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Multireference space without first solving the configuration interaction problem
Authors:Vitaly N Glushkov  Xavier Assfeld
Institution:1. Department of Physics, Electronics and Computer Systems, Oles Gonchar Dnipropetrovsk National University, Dnipropetrovsk, Ukraine;2. Théorie‐Modélisation‐Simulation, UMR CNRS 7565, Faculté des Sciences et Technologies, Université de Lorraine, Vandoeuvre‐lès‐Nancy, France
Abstract:We further develop an idea to generate a compact multireference space without first solving the configuration interaction problem previously proposed for the ground state (GS) (Glushkov, Chem. Phys. Lett. 1995, 244, 1). In the present contribution, our attention is focused on low‐lying excited states (ESs) with the same symmetry as the GS which can be adequately described in terms of an high‐spin open‐shell formalism. Two references Møller–Plesset (MP) like perturbation theory for ESs is developed. It is based on: (1) a main reference configuration constructed from the parent molecular orbitals adjusted to a given ES and (2) secondary double excitation configuration built on the GS like orbitals determined by the Hartree–Fock equations subject to some orthogonality constraints. It is shown how to modify the MP zeroth‐order Hamiltonian so that the reference configurations and corresponding excitations are eigenfunctions of it and are compatible with orthogonality conditions for the GS and ES. Intruder states appearance is also discussed. The proposed scheme is applied to the GS, ES, and excitation energies of small molecules to illustrate and calibrate our calculations. © 2013 Wiley Periodicals, Inc.
Keywords:multireference perturbation theory  correlation energy  excited states  orthogonality constraints
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